1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a zoom lens system for lens shutter cameras with a built-in zoom lens system.
2. Description of the Prior Art
In lens shutter cameras with a built-in zoom lens system, in order to achieve a camera with compactness and low cost, it is desirable to make the photo-taking lens system itself compact and low-cost. As far as compactness is concerned, priority is placed more on compactness when the lenses are being stored in the camera than on the compactness when they are in a photo-taking position (wide-angle or telephoto position). For this purpose, it is necessary to reduce the number of lenses used, as well as to make each lens unit thinner by reducing the thickness of each lens and shortening the distances between lenses.
From this perspective, to achieve compactness, a zoom lens system which comprises three lens units, i.e., a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power, aligned in that order from the object side, and which changes magnification via moving each lens unit, has been proposed in, for example, Patent Gazettes H2-201409, H3-265811, H4-123014 and H4-153613.
However, while the zoom lens systems proposed in the above Patent Gazettes H2-201409 and H3-265811 use a small number of lenses relative to the zoom ratio, they do not meet current needs for compactness and low cost. In the zoom lens system proposed in H2-201409 in particular, because the structure is such that the number of lenses in the first and third lens units is relatively small, but the number of lenses in the second lens unit is large, its cost turns out to be high. In addition, in the zoom lens systems proposed in H4-123014 and H4-153613, although the number of lenses is very small and the thickness of each lens unit is small, chromatic aberrations are not sufficiently corrected.